In the past, in a sensor for detecting the concentration of a particular gas component in a gas to be measured, such as an exhaust gas, as shown in Japanese Laid-Open Patent Application Publication 2002-181765 (“JP ‘765”) a detection element is provided at the bottom part of the gas sensor, and a through hole is formed so as to pass through the center part of a seal member provided at the top part of the gas sensor in the axial direction, and further a ventilation filter is provided so as to block the through hole. This ventilation filter, in addition to introducing outside air into the inside of the gas sensor, is ventilating and water-repellant, so that even if the gas sensor is subjected to water, water is not introduced into the inside of the gas sensor. In this gas sensor, the sheet-like sealed ventilation filter is fitted over a hollow-cylindrical member and inserted into the through hole, so as to be held inside the through hole between the outer peripheral surface of the hollow-cylindrical member and the inner peripheral surface of the through hole, thereby blocking the through hole and achieving ventilation and waterproofing thereof.
An oxygen sensor such as in JP ‘765, however, uses PTFE or a fluorine-based resin or the like as a material, is susceptible to externally applied impacts, is damaged by externally applied force, and there is a risk of a hole forming in the ventilation filter. As a result, seepage of water content into the inside of the gas sensor from the outside can cause such problems as a decrease in the oxygen concentration detection output, or splitting of the detection element by being subjected to water, these being causes of gas sensor failure.
The present invention was made in view of the problems of the past, and provides a gas sensor having a high-strength ventilation filter while maintaining the ventilation and water repellency properties of the ventilation filter.